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Modbus or MODBUS is a client/server data communications protocol in the application layer. [1] It was originally designed for use with programmable logic controllers (PLCs), [2] but has become a de facto standard communication protocol for communication between industrial electronic devices in a wide range of buses and networks.
A remote terminal unit (RTU) is a microprocessor-controlled electronic device that interfaces objects in the physical world to a distributed control system or SCADA (supervisory control and data acquisition) system by transmitting telemetry data to a master system, and by using messages from the master supervisory system to control connected objects. [1]
RS-485 is used as the physical layer underlying many standard and proprietary automation protocols used to implement industrial control systems, including the most common versions of Modbus and Profibus. DH 485 is a proprietary communications protocol used by Allen-Bradley in their line of industrial control units.
The DNP User Group has established four levels of subsets of the protocol for RTU compliance. The DNP Users Group has published test procedures for Levels 1 and 2, the simplest implementations. The protocol is robust, efficient, and compatible with a wide range of equipment, but has become more complex and subtle over time.
Many are designed to send information only when the master station polls the RTU. Typical legacy SCADA protocols include Modbus RTU, RP-570, Profibus and Conitel. These communication protocols, with the exception of Modbus (Modbus has been made open by Schneider Electric), are all SCADA-vendor specific but are widely adopted and used.
Image credits: Photoglob Zürich As evident from Niépce's and Maxwell's experiments, and as photographic process historian Mark Osterman told Bored Panda, the processes behind colored photographs ...